Dipankar Choudhury

Effects of surface coating on reducing friction and wear of orthopaedic implants

Abstract Coatings such as diamond-like carbon (DLC) and titanium nitride (TiN) are employed in joint implants due to their excellent tribological properties. Recently, graphite-like carbon (GLC) and tantalum (Ta) have been proven to have good potential as coating as they possess mechanical properties similar to bones—high hardness and high flexibility. The purpose of this systematic literature review is to summarize the coating techniques of these four materials in order to compare their mechanical properties and tribological outcomes. Eighteen studies published between January 2000 and February 2013 have met the inclusion criteria for this review. Details of their fabrication parameters, material and mechanical properties along with the tribological outcomes, such as friction and wear rate, were identified and are presented in a systematic way. Although experiment conditions varied, we conclude that Ta has the lowest wear rate compared to DLC, GLC and TiN because it has a lower wear rate with high contact pressure as well as higher hardness to elasticity ratio. However, a further tribology test is needed in an environment which replicates artificial joints to confirm the acceptability of these findings.

Tribological investigation of diamond-like carbon coated micro-dimpled surface under bovine serum and osteoarthritis oriented synovial fluid

Abstract Osteoarthritis-oriented synovial fluid (OASF), i.e., that typical of a patient with osteoarthritis, has different physical and biological characteristics than bovine serum (BS), a lubricant widely used in biotribological investigations. Micro-dimpled and diamond-like carbon- (DLC) coated surfaces are key emerging interfaces for orthopedic implants. In this study, tribological performances of dimpled surfaces, with and without DLC coating, have been investigated under both BS and OASF. The friction tests were performed utilizing a pin on a disk tribometer, whereas contact pressure, speed, and temperature were simulated to a ‘medium walking gait’ of hip joint conditions. The mechanical properties of the specimen and the physical properties of the lubricant were characterized before the friction test. Raman analysis was conducted to identify the coating condition both before and after the test. The DLC-coated dimpled surface showed maximum hardness and residual stress. A DLC-coated dimpled surface under an OASF lubricated condition yielded a lower friction coefficient and wear compared to those of plain and dimpled specimens. The higher graphitization of coated materials with increasing load was confirmed by Raman spectroscopy.

Fabrication and characterization of DLC coated microdimples on hip prosthesis heads

Diamond like carbon (DLC) is applied as a thin film onto substrates to obtain desired surface properties such as increased hardness and corrosion resistance, and decreased friction and wear rate. Microdimple is an advanced surface modification technique enhancing the tribological performance. In this study, DLC coated microdimples were fabricated on hip prosthesis heads and their mechanical, material and surface properties were characterized. An Electro discharge machining (EDM) oriented microdrilling was utilized to fabricate a defined microdimple array (diameter of 300 µm, depth of 70 µm, and pitch of 900 µm) on stainless steel (SS) hip prosthesis heads. The dimpled surfaces were then coated by hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (Ta-C) layers by using a magnetron sputtering technology. A preliminary tribology test was conducted on these fabricated surfaces against a ceramic ball in simulated hip joint conditions. It was found that the fabricated dimples were perpendicular to the spherical surfaces and no cutting-tools wear debris was detected inside the individual dimples. The a-C:H and Ta-C coatings increased the hardness at both the dimple edges and the nondimpled region. The tribology test showed a significant reduction in friction coefficient for coated surfaces regardless of microdimple arrays: the lowest friction coefficient was found for the a-C:H samples (µ = 0.084), followed by Ta-C (µ = 0.119), as compared to the SS surface (µ = 0.248).

Diamond-like carbon coatings with zirconium-containing interlayers for orthopedic implants

Six types of diamond-like carbon (DLC) coatings with zirconium (Zr)-containing interlayers on titanium alloy (Ti-6Al-4V) were investigated for improving the biotribological performance of orthopedic implants. The coatings consist of three layers: above the substrate a layer stack of 32 alternating Zr and ZrN sublayers (Zr:ZrN), followed by a layer comprised of Zr and DLC (Zr:DLC), and finally a N-doped DLC layer. The Zr:ZrN layer is designed for increasing load carrying capacity and corrosion resistance; the Zr:DLC layer is for gradual transition of stress, thus enhancing layer adhesion; and the N-doped DLC layer is for decreasing friction, squeaking noises and wear. Biotribological experiments were performed in simulated body fluid employing a ball-on-disc contact with a Si3N4 ball and a rotational oscillating motion to mimic hip motion in terms of gait angle, dynamic contact pressures, speed and body temperature. The results showed that the Zr:DLC layer has a substantial influence on eliminating delamination of the DLC from the substrates. The DLC/Si3N4 pairs significantly reduced friction coefficient, squeaking noise and wear of both the Si3N4 balls and the discs compared to those of the Ti-6Al-4V/Si3N4 pair after testing for a duration that is equivalent to one year of hip motion in vivo.

Tribological performance of the biological components of synovial fluid in artificial joint implants

Abstract The concentration of biological components of synovial fluid (such as albumin, globulin, hyaluronic acid, and lubricin) varies between healthy persons and osteoarthritis (OA) patients. The aim of the present study is to compare the effects of such variation on tribological performance in a simulated hip joint model. The study was carried out experimentally by utilizing a pin-on-disk simulator on ceramic-on-ceramic (CoC) and ceramic-on-polyethylene (CoP) hip joint implants. The experimental results show that both friction and wear of artificial joints fluctuate with the concentration level of biological components. Moreover, the performance also varies between material combinations. Wear debris sizes and shapes produced by ceramic and polyethylene were diverse. We conclude that the biological components of synovial fluid and their concentrations should be considered in order to select an artificial hip joint to best suit that patient.

Tribological investigation of ultra-high molecular weight polyethylene against advanced ceramic surfaces in total hip joint replacement

The aim of the study was to investigate whether a modified ceramic head surface could reduce the friction and wear rate of simulated ceramic-on-polyethylene hip joints. To address this aim, ultra-high molecular weight polyethylene (UHMWPE) was made to slide on aluminium oxide (Al2O3), dimpled Al2O3, diamond-like carbon (DLC) coated and DLC-coated dimpled substrates. The experiment condition was replicated to simulate artificial hip joints in terms of contact pressure, speed and temperature. UHMWPE on non-dimpled Al2O3 showed lower friction coefficient and wear rate compared to other advanced surfaces. Lower wettability, and higher hardness and surface adhesion of DLC resulted in increased friction and wear. The high difference in modulus of elasticity and hardness between UHMWPE and both, Al2O3 and DLC, reduced the effectiveness of textured surface techniques in friction and wear reduction. Therefore, no tribological benefit was found by fabricating either DLC coating or surface texturing on hard surface when rubbed against softer UHMWPE.

Lubricating ability of albumin and globulin on artificial joint implants: a tribological perspective

Albumin and globulin proteins are dominant protein components in synovial fluid, and play important roles in the lubrication mechanism of joint prostheses. The present study investigated the lubricating ability of albumin and globulin on ceramic-on-polyethylene joint implants. A pin-on-disk tribometer was used for friction tests where the experimental condition was replicated with a simulated hip joint condition. The experiment was conducted under three lubricating conditions: bovine synovial fluid (BSF), albumin and globulin. Mechanical properties of specimens and physical properties of lubricant were measured before and after the tests. The experimental results show that albumin yielded a lower friction coefficient as compared to globulin. However, both exhibited a higher friction coefficient and wear rate than that of bovine synovial fluid. This study suggests that further tribological investigations on major biological components of synovial fluid should be carried out, which will help optimise implant design.

Mechanical Wear and Oxidative Degradation Analysis of Retrieved Ultra High Molecular Weight Polyethylene Acetabular Cups

The number of revision joint replacements has been increasing substantially over the last few years. Understanding their failure mechanism is extremely important for improving the design and material selection of current implants. This study includes ten retrieved and four new mildly cross-linked ultra-high molecular weight polyethylene (UHMWPE) acetabular liners. Among them, most of the prostheses (n = 5) were reported to be revised and replaced due to aseptic loosening, followed by painful joint (n = 2), dislocation (n = 1), intra articular ossification (n = 1), combination of wear (liner) and osteolysis (stem) (n=1). Surface deviations (wear, material inflation and roughness), oxidative degradation and change of material properties were measured using micro-computed tomography (micro-CT) scan, 3D laser scanning microscopy, raman spectroscopy and nanoindentation, respectively. Prostheses having eccentric worn areas had much higher linear wear rates (228.01 ± 35.51µm/year) compared to that of centrically worn prostheses (96.71 ± 10.83µm/year). Oxidation index (OI) showed similar trends to the surface penetration depth. Among them, sample 10 exhibited the highest OI across the contact area and the rim of the cup liner. It also had the lowest hardness/elasticity ratio. Overall, wear and creep, oxidative degradation and reduced hardness/elasticity ratio all contributed to the premature failure of the UHMWPE acetabular cup liners.

Enhanced lubricant film formation through micro-dimpled hard-on-hard artificial hip joint: An in-situ observation of dimple shape effects

This study evaluates the impact of dimple shapes on lubricant film formation in artificial hip joints. Micro-dimples with 20-50 µm lateral size and 1 ± 0.2 µm depths were fabricated on CrCoMo hip joint femoral heads using a picosecond laser. Tribological studies were performed using a pendulum hip joint simulator to apply continuous swing flexion-extension motions. The results revealed a significantly enhanced lubricant film thickness (≥ 500 nm) with micro-dimpled prosthesis heads at equilibrium position after the lubricant film has fully developed. The average lubricant film thickness of dimpled prostheses with square- and triangular-shaped dimple arrays over time is about 3.5 that of the non-dimpled prosthesis (204 nm). Remarkably, the prosthesis with square-shaped dimple arrays showed a very fast lubricant film formation reaching their peak values within 0.5 s of pendulum movement, followed by prosthesis with triangular-shaped dimple arrays with a transition period of 42.4 s. The fully developed lubricant film thicknesses (≥ 700 nm) are significantly higher than the surface roughness (≈ 25 nm) demonstrating a hydrodynamic lubrication. Hardly any scratches appeared on the post-experimental prosthesis with square-shaped dimple array and only a few scratches were found on the post-experimental prosthesis with triangular-shaped dimple arrays. Thus, prostheses with square-shaped dimple arrays could be a potential solution for durable artificial hip joints.

The Influence of Surface Modification on Friction and Lubrication Mechanism Under a Bovine Serum–Lubricated Condition

ABSTRACT Diamond-like carbon (DLC) and microdimples are two potential surface modification techniques that are extensively studied to be utilized in biotribological interfaces in order to reduce the friction coefficient and wear rate. However, in situ observation of bovine serum–lubricated DLC and microdimpled surface contacts are not well understood. In this study, a DLC-coated and a microdimpled steel ball rubbing against a Cr-coated glass disk, where 25% bovine serum was used as a lubricant and the temperature was maintained at 37°C, were investigated. The behaviors of ithe nterface were ca`ptured using optical interferometry and the friction coefficients were simultaneously measured using a torque sensor. The experimental results reveal that DLC/glass sliding is scuffing-free, with a lower friction coefficient; however, the formation of a lubricating film is insignificant. On the other hand, the dimples retained lubrication and, as a result, the wear of the glass disk was minimized; however, the friction coefficient was not reduced. Therefore, DLC and microdimples individually have few improved tribological features, but their combination should be considered to maximize performance.